Research suggests late adolescence and emerging adulthood (EA; roughly encompassing ages 18-25) are associated with increased risk for development of substance misuse, abuse, and dependence. Despite increased risk overall during this period, there is considerable variability in drinking trajectories during young adulthood (Schulenberg et al., 1996; Tucker et al., 2005). Much of the research on alcohol use and prevention during EA has focused on students who matriculate into college during this period (Larimer & Cronce, 2002), with relatively little research focused on etiology and prevention of alcohol use among individuals who do not immediately matriculate to college (White et al., 2005). Further, little research has compared trajectories of alcohol use and consequences in international samples, including the individual, social, and environmental/cultural factors influencing trajectories and transitions in alcohol use patterns. Finally, almost no research has systematically evaluated efficacy of preventive interventions offered prior to the transition from high school as a means of altering trajectories during EA. The current research is designed to address these gaps, through a longitudinal study of a cohort of high school students in both the US and Sweden, and the evaluation of web-based interventions offered to a random subset (n=400 per country) at the end of high school as a means of reducing or preventing excessive alcohol consumption and related consequences. Specific aims of the current application are: 1) Evaluate intrapersonal, peer, parental, environmental, and cultural predictors of alcohol use trajectories upon the transition from high school in US and Swedish populations, guided by the Theory of Triadic Influence; 2) Evaluate the efficacy of a web-based interactive feedback and skills intervention in reducing alcohol use and negative consequences over a four- year follow-up period; 3) Evaluate moderators of intervention efficacy, including individual-level and cultural factors; and 4) Evaluate mediators of intervention efficacy. Aims will be achieved through a longitudinal follow-up survey of 2400 teens across 2 sites: the King Country region of Washington State and the Skane region of Sweden. Teens will be recruited during their senior year in high school and assessed twice per year for 4 years. RELEVANCE (See instructions); Alcohol misuse during emerging adulthood is an international public health problem. This research evaluates trajectories and prevention of alcohol use, using the framework of the Theory of Triadic Influence to evaluate transitions into and out of heavy drinking across emerging adulthood. The international comparison allows for disentangling of effects of development from other factors including matriculation into college and attaining legal drinking age, and impacts of distal cultural factors which vary between the US and Sweden.